This invention relates generally to coatings that emit magnetic resonance signals and in particular, to such coatings containing paramagnetic metal ions, and to a process for coating medical devices with such coatings so that the devices are readily visualized in magnetic resonance images during diagnostic or therapeutic procedures done in conjunction with magnetic resonance imaging (MRI).
Since its introduction, magnetic resonance (MR) has been used to a large extent solely for diagnostic applications. With advancement of magnetic resonance imaging, however, it is becoming possible to replace many diagnostic x-ray imaging applications with MR techniques. For example, the accepted standard for staging vascular disease was, at one time, x-ray contrast angiography. Today, MR angiographic techniques are being increasingly used to detect vascular abnormalities and, in some specific clinical instances, contrast-enhanced MR angiograms are rapidly approaching the diagnostic standard set by x-ray angiography.
More recently, advances in MR hardware and imaging sequences have begun to permit the use of MR in certain therapeutic procedures. That is, certain therapeutic procedures or therapies are performed on a patient while the patient and the instruments, devices or agents used and/or implanted are being imaged. The use of MR in this manner of image-guided therapy is often referred to as interventional magnetic resonance (interventional MR). These early applications have included: monitoring ultrasound and laser ablations, guiding the placement of biopsy needles, and visualizing disease, such as tumors, interoperatively.
Of particular interest in interventional MR is endovascular therapy. Endovascular therapy refers to a general class of minimally-invasive interventional (or surgical) techniques which are used to treat vascular abnormalities. Unlike conventional surgical techniques, endovascular therapies access and treat the disease from within the vasculature. The vascular system is usually accessed via the femoral artery. A small incision is made in the groin and the femoral artery punctured. A sheath is then inserted for vascular access. A catheter with the addition of a guide-wire can then be manipulated under fluoroscopic guidance to the area of interest. The guide-wire is then removed from the catheter lumen, and either a therapeutic device (e.g., balloon, stent, coil) is inserted with the appropriate delivery device, or an agent (e.g., embolizing agent, anti-vasospasm agent) is injected through the catheter. In either instance, the catheter functions as a conduit and ensures the accurate and localized delivery of the therapeutic device or agent. Once the device or agent is in place, its delivery system is withdrawn, i.e., the catheter is withdrawn, the sheath removed and the incision closed. The duration of an average endovascular procedure is about 3 hours, although difficult cases may take more than 8 hours. Traditionally, such procedures have been performed under x-ray fluoroscopic guidance.
Performance of these procedures under MR-guidance provides a number of advantages. Safety issues are associated with the relatively large dosages of ionizing radiation required in x-ray fluoroscopy. While radiation risk to the patient is of somewhat less concern (since it is more than offset by the potential benefit of the procedure), exposure to the interventional staff can be a major problem. In addition, the complication rate from MR contrast agents is much less than the commonly used iodinated x-ray contrast agents.
Other advantages of MR-guided procedures include the ability of MR to acquire three-dimensional images. In contrast, most x-ray angiography systems can only acquire a series of projection images. MR has clear advantages when multiple projections or volume reformatting are required in order to understand the treatment of complex three-dimensional vascular abnormalities, such as arterial-venous malformations (AVMs) and aneurysms. Furthermore, MR is sensitive to a variety of “functional” parameters including temperature, blood flow, tissue perfusion, diffusion and brain activation. This additional diagnostic information, which, in principle, may be obtained before, during and immediately after therapy, cannot be acquired by x-ray fluoroscopy alone. It is likely that once suitable MR-based endovascular procedures have been developed, the next challenge will be to integrate this functional information with conventional anatomical imaging and device tracking.
Currently, both “active” and “passive” approaches are being used to monitor the placement of interventional devices under MR guidance. With active tracking, visualization is accomplished by incorporating one or more small radio-frequency (RF) coils into the device, e.g., a catheter. The position of the device is computed from MR signals detected by the coil. Later, this information is superimposed on a previously acquired anatomical “road map” image. The advantages of active tracking include excellent temporal resolution and spatial accuracy, and the ease with which the tip position, e.g., of a catheter, can be updated at 20 Hz, i.e., 20 times per second.
However, active methods allow visualization of only a discrete point(s) on the device. Typically, only the tip of the device is “active”, i.e., visualized. Although it is possible to incorporate multiple RF coils (4-6 on typical clinical MR systems) into a device, it is still impossible to determine position at more than a few discrete points along the device. While this may be acceptable for tracking rigid biopsy needles, this is a significant limitation for tracking flexible devices as in endovascular therapy. Furthermore, intravascular heating due to RF-induced currents is a concern with active methods.
As noted above, the attachment of coils onto flexible catheters present numerous challenges. Also, the effect on the mechanical properties of catheters is of concern. Ladd et al. (Ladd et al., Proc. ISMRM (1997) 1937) have addressed some of the deficiencies of an active catheter by designing an RF coil that wraps about the catheter. This allows visualization of a considerable length of a catheter, but still does not address the problems of RF heating and mechanical catheter performance.
Passive tracking technologies use the fact that endovascular devices do not generally emit a detectable MR signal and, thus, result in areas of signal loss or signal voids in MR images. Such signal loss, for example, occurs with a polyethylene catheter. By following the void, the motion of the catheter can be inferred. One advantage of passive tracking methods over active methods is that they do allow “visualization” of the entire length of a device. Signal voids, however, are certainly not optimal for device tracking because they can be confused with other sources of signal loss.
A further source of passive contrast occurs if the device has a magnetic susceptibility much different than tissue (e.g., metallic guide-wires and stents). Susceptibility differences cause local distortions to the magnetic field and result in regions of signal enhancement and of signal loss surrounding the device. A number of published reports describe passive catheter visualization schemes based on signal voids or susceptibility-induced artifacts. A principal drawback of the currently available passive techniques is that visualization is dependent on the orientation of the device with respect to the main magnetic field.
Despite recognition and study of various aspects of the problems of visualization of medical devices in therapeutic, especially endovascular, procedures, the prior art has still not produced satisfactory and reliable techniques for visualization and tracking of the entire device in a procedure under MR guidance.